Polarizing plates currently used in image display units such as liquid crystal displays commonly have triacetyl cellulose films (hereinafter, TAC films) included therewith as protective films, in order to protect a polyvinyl alcohol polarizer. However, TAC films may not have a sufficient degree of heat and humidity resistance, and may therefore have a problem in that the properties of polarizing plates, such as polarization degree or color, may be degraded due to film deformation when used under conditions of high temperature and high humidity. Accordingly, the use of transparent acrylic resin films having excellent heat and humidity resistance, instead of TAC films, has recently been proposed for polarizer protective films.
Meanwhile, technologies in which an ultraviolet ray absorbent is added to a protective film in order to protect a polyvinyl alcohol polarizer from ultraviolet light have also been recently proposed. For ultraviolet light within sunlight, a wavelength band of 320 to 400 nm is referred to as the UVA region, and a wavelength band of 290 to 320 nm is referred to as the UVB region. Light from within the UVA region is not absorbed in the ozone layer and also has high intensity, so thus should be blocked without fail, and although light from within the UVB region is mostly absorbed in the ozone layer, light from within the UVB region needs to be blocked, even in the case that the amount reaching the ground is negligible, since this light has a short wavelength and accordingly, a relatively large amount of energy. However, ultraviolet light absorbent-including protective films that have been proposed to date have problems, in that the ultraviolet light region able to be blocked is limited to light within a wavelength band of 320 to 400 nm.
In addition, in ultraviolet light absorbent-including acrylic optical films that have been proposed so far, a considerable amount of the ultraviolet light absorbent needs to be added in order to achieve a meaningful ultraviolet light blocking effect, however, when a considerable amount of the ultraviolet light absorbent is added, migration, a phenomenon in which an ultraviolet light absorbent strains a casting roll as it is decomposed and escapes from the film when the acrylic resin, melted by high temperature and pressure from an extruder, abruptly goes cold while passing through a T-die and then a casting roll, may be a serious problem in an acrylic film preparation process, and as a result, surface qualities of the exterior of the film may be poor due to the heat decomposed-ultraviolet light absorbent also straining the film. Furthermore, an ultraviolet light absorbent has a low molecular weight and a low glass transition temperature, therefore, when the content of the ultraviolet light absorbent increases, the molecular weight and the glass transition temperature of an acrylic film including this absorbent also decrease, resulting in the decrease of heat resistance.
Furthermore, in acrylic optical films including ultraviolet light absorbents that has been proposed to date, a portion of the ultraviolet light absorbent included therein may be broken by ultraviolet light resulting in the generation of radicals when such acrylic optical films are exposed to ultraviolet light for a long period of time, therefore, allowing for the absorption of light from within regions outside of the region that the ultraviolet light absorbent absorbs initially. As a result, optical transmittance in some wavelength bands may be greatly decreased, causing a yellowing phenomenon in which the film is discolored to have a yellow hue.
Therefore, an acrylic optical film capable of effectively blocking ultraviolet light in wavelength bands of 320 to 400 nm (UVA region) and ultraviolet light in wavelength bands of 290 to 320 nm (UVB region) even in the case that a small amount of an ultraviolet ray absorbent is included, and furthermore, having a stable optical spectrum in which almost no changes in optical transmittance are observed even when exposed to ultraviolet light for a long period of time, is required.